This invention relates to the detection of prerecorded servo track positioning information, and, more particularly, to the detection and track following of servo track positioning information from servo tracks having at least two edges, each edge comprising an interface between two dissimilar recorded servo signals, as read by a servo read element, to allow positioning of the servo read element and the corresponding data head.
In the data storage industry, advances in technology include increases in the data storage capacity of given data storage media. One means of increasing the data storage capacity of data storage media, such as magnetic tape cartridges or magnetic tape cassettes, is to increase the track density of the data storage media, and in a corresponding manner, to decrease the width of each track.
In a typical magnetic tape, data is recorded in a plurality of parallel, longitudinal data tracks. A data head may have a plurality of data heads which have fewer numbers of read/write elements than tracks. The data tracks are divided into groups, typically interleaved, and the data head is indexed laterally with respect to the tracks to access each group.of data tracks. In order to properly register the data head with the data tracks, prerecorded servo tracks are provided which are parallel to the data tracks. A servo read head located at an indexed position with respect to the read/write elements reads the servo tracks. The servo tracks provide lateral positioning information which, when read by the servo read head, can be decoded to indicate whether the servo read head is correctly positioned with respect to the servo tracks. Thus, the servo head can be moved laterally to a desired position with respect to the servo tracks so as to properly register the read/write elements with respect to a desired group of data tracks. Then, the servo head can follow the servo tracks as the media and the head are moved longitudinally with respect to each other, so that the read/write elements maintain registration with the data tracks.
As an example, the prerecorded servo track positioning information comprises adjoining servo tracks having different servo patterns, one of the servo patterns comprising a constant amplitude signal of a single first frequency, and the other servo pattern alternating between a constant amplitude burst signal of a single second frequency and a zero amplitude null signal. The interface between the adjoining servo signals is called an xe2x80x9cedgexe2x80x9d. The resultant signal read by the servo head is a maximum signal comprising the first frequency signal combined with the second frequency burst signal and a minimum signal comprising the first frequency signal combined with the null signal. If the servo head is correctly positioned at the junction of the adjoining servo tracks, the amplitude of the combined first and second frequency signals is twice the amplitude of the combined first and null signals, and is easily decoded. Coassigned U.S. Pat. No. 5,448,430 illustrates the above discussed servo track patterns and describes a track following servo positioning system employing peak detection to determine the maximum and minimum signals.
As data capacity is increased, it is also desirable to have backward compatibility to data storage media having the prior level of data capacity, to avoid the necessity of copying all of the data recorded on the prior media onto the new media.
It is therefore an object of the present invention to increase the track density of a data storage media, while utilizing the prior media servo tracks, so that the servo system may be operated to utilize the servo track positioning information as before for the prior media, and to utilize the same servo track positioning information in a more precise manner to access tracks at a higher track density.
A servo position detector and a method for detecting servo positioning with respect to an index position displaced at an offset with respect to a servo track are provided. The servo track has at least two edges, each edge comprising an interface between two dissimilar recorded servo signals. The edges are on opposite lateral sides of a middle recorded servo signal, and the edges are separated by a predetermined distance.
The sensor comprises a servo read element having an active sensing region which is no more than and is substantially the entire predetermined distance separating the edges. Thus, the servo read element thereby senses no more than two of the dissimilar recorded servo signals and at one of the edges. Logic coupled to the servo read element compares the two sensed servo signals to determine the ratio therebetween, and determines an error between the compared ratio of the sensed servo signals and a predetermined ratio. The predetermined ratio comprises a predetermined offset from the centered on-edge ratio of amplitudes in a predetermined direction, and identifies the index position. The index position is substantially parallel to the edge and displaced from the edge.
The logic provides an output signal related to the determined error, the output signal identifying the servo position error with respect to the index position.
In further embodiments of the invention, a servo track follower and method are provided for detecting and following the servo index position related to the servo track. A servo positioner is coupled to the logic and moves the servo read element in a direction to reduce the servo position error.
In another embodiment of the invention, the displacement of the index position from one of the edges is substantially 1/4 of the predetermined distance separating the edges, for a total of four index positions, one on each side of an edge. Another embodiment employs the edges as in the prior art and additionally has index positions displaced substantially 1/3 of the predetermined distance separating the edges, for a total of six index positions. The servo read element active sensing region distance comprises between substantially 80% and substantially 100% of the predetermined distance separating the edges.
In still another embodiment of the invention, for detecting the servo positioning with respect to a plurality of parallel spaced servo tracks, a plurality of the servo read elements are spaced apart to sense the two dissimilar signals at each corresponding edge of the servo tracks. The logic additionally averages the corresponding sensed two dissimilar signals of each of the plurality of read elements and determines an error between the compared ratio of the averaged dissimilar servo signals and the predetermined ratio.
In a further embodiment of the invention, an independent position sensor is additionally provided for sensing the coarse position of the servo read element with respect to a reference, the coarse position indicating which of the edges of the servo track is aligned with and detected by the servo read element. The logic additionally employs the coarse position to identify the predetermined index position and to select the predetermined ratio.
An input receives the input signal, and logic is coupled to the input. The logic responds to the received input signal to determine the identified index position, selecting the edge and a predetermined ratio representing the input indicated predetermined index position, the predetermined ratio comprising a predetermined offset from the centered on-edge ratio in a predetermined direction. A servo read element having an active sensing region extends substantially orthogonal to the one of the edges to sense no more than two of the dissimilar recorded servo signals at one of the edges. An independent position sensor senses the present coarse position of the servo read element with respect to a reference, the present coarse position indicating the present one of the predetermined index positions of the servo track that is aligned with and detected by the servo read element. The logic is coupled to the independent position sensor and to the servo read element. The logic responds to the present coarse position to indicate the present index position detected by the servo read element. The logic responds to the present index position and to the input identified index position, first operating a positioning servo in a direction from the present index position toward the input identified index position, thereby selecting the input identified one of the predetermined index positions. The logic then compares the two sensed servo signals to determine a ratio therebetween, and determines an error between the ratio of the compared sensed servo signals and the selected predetermined ratio of the selected one of the predetermined index positions, thereby determining a servo position error with respect to that index position. The logic provides a position error output to the positioning servo to operate the positioning servo in a direction to reduce said servo position error, thereby track following the selected predetermined index position.